    
There is a third rather weighty argument in favor of causality. We study relativistic quantum systems endowed with a WL for which, by definition, there is no transmission of signals faster than light, thus implementing Einstein causality. Considering only physical systems, viz. systems with non-negative mass-squared operator and finite spinor dimension, there are exactly three such irreducible systems, the Dirac system and the two Weyl systems.Actually we did not succeed as yet to rule out certain irreducible massive and massless SCT, see (REF ), (REF ). This proves uniqueness and fundamental importance of the Dirac system for massive quantum systems. In view of the recently discovered Weyl fermions (see [1]}, [2]} 2015) this may equally hold true for the Weyl systems with regard to massless quantum systems. One notes that the requirement of a causal localization also predicts the opposite handedness of particle and antiparticle constituting a Weyl system.
First, however, the above classification shows the far-reaching impact of the principle of causality.
